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A few things could happen, though:
-Your Water Type is too weak to survive another hit (or worse, KO'd)
-You could be Burned by WoW or a damaging Fire-Type Move on a switch
-Pokemon with Intimidate could drop your Attack a few times and possibly carry Fire Type Moves (Arbok, Arcanine, Tauros, Mawile, Salamence, Staraptor*, Luxray, Stoutland)
-You face a high Defense-low Sp.Def Fire Type (Torkoal or Magcargo)
-You face a high Defense-low Sp.Def Pokemon that resists Bug Type Moves in-general (Aggron, Skarmory, Steelix, Golem, Gigalith)
-Cofagrigus enters hoping to screw you out of further Bug-Type Move boosts or Critical Hit blocking
-Counter and Metal Burst users could 1KO you from Megahorn use
-You could miss entirely (Megahorn's accuracy is 85% and you could be further hindered by Muddy Water, Mud Bomb, or a lucky enemy's Acupressure)

Protecting your Attack from hindrance is as important as KOing any Pokemon that can hinder it. Without your key offensive stat, you will have a ton of issues. Rain + HP (Water) is a safety net.

Simple math: HP water + rain = 140 base power (assuming perfect IV which would not be the case outside of PO) * 2 = 280 base power against something SE on it coming from a pitiful base 60 special attack and mosly uninverted meaning it will be like 150 special attack at lvl 100...

VS

non effective STAB megahorn (which you can further boost with swords dance) 180/2= 90 base power from 135 base attack stat his attack stat is mostly inverted and have a chance to be boosted to 180 power again
85 accuracy is more than acceptable (8.5/10 chance to hit).

turn 1: you switch in excavalier in choiced latios with draco meteor (or what ever)

turn 2: they switch to volcarona then you swords dance

turn 3: they use quiver dance cause it sounds stupid that you will stay then you KO with megahorn.. (and if there is stealth rock better)

They've just effectively negated the Choice Band. They still have to deal with a 120 Base Power STAB attack coming off a Base 135 attack. Besides, of those, only Arcanine is afraid of Water attacks, and it still kills you anyways.

Last time I checked, wasn't escavalier in UU... I mean, who in the world would want to use it in the OU tier, where any unit with even the slightest bit of special attack uses HP Fire cuz of scizor and ferrothorn, never the less, the over-abundance of highly effective fire types in general in the tier

While still on the Mixcavalier discussion, I would note that maxing out special attack would mean less EVs in HP, so it probably won't survive as long.

Golem and Gigalith do not resist Bug attacks, and of the others only Skarmory is common. And it's not even Water weak.

I still think mixed or specs escavalier is a bad idea, but Golem is part ground type, so it resist Bug.

Last time I checked, wasn't escavalier in UU... I mean, who in the world would want to use it in the OU tier, where any unit with even the slightest bit of special attack uses HP Fire cuz of scizor and ferrothorn, never the less, the over-abundance of highly effective fire types in general in the tier

Though it's the tier with politoed for support, and POTW focuses on OU. Sure, fire attacks are everywhere, but same thing with bulky waters. A good team in OU needs a fire counter.